Optically variable devices of various types are used as efficient anti-copying means on security- and value-documents. Among these, particularly important optically variable copy-protection means are the optically variable inks (OVI®; EP-A-0227423). Optically variable inks (OVI®) are used to print surfaces and/or indicia which exhibit a viewing angle dependent color (=color shift).
Said anti-copying inks are formulated on the basis of optically variable pigments (OVP); preferred types being the flake shaped thin-film optical interference pigments described in U.S. Pat. No. 4,705,300; U.S. Pat. No. 4,705,356; U.S. Pat. No. 4,721,271 and thereto related disclosures.
Other useful types of OVP for optically variable ink formulations comprise the interference coated particles described in U.S. Pat. No. 5,624,486 and U.S. Pat. No. 5,607,504, and the thin-film cholesteric (i.e. chiral-nematic) liquid crystal pigments described in U.S. Pat. No. 5,807,497 and U.S. Pat. No. 5,824,733.
Optically variable inks, coatings and paints are known in the art, e.g. from EP-A-0227423, U.S. Pat. No. 5,279,657, or WO 95/29140. Said optically variable inks can be used in various printing processes, such as copperplate (Intaglio) printing, gravure printing, flexographic printing or screen printing.
As known to the skilled person, the wet film thickness resulting from the said printing processes can vary to a large extent, form about 2 μm up to about 50 μm depending upon the process and the conditions used.
To achieve a high color-shifting effect of the optically variable ink or coating, the shape of the optically variable pigment (OVP) is preferably a platelet or a flake, as disclosed in the art.
The perceived optical characteristics and the color purity depend on the pigment's final orientation in the cured ink or coating layer on the substrate. Randomly oriented optically variable pigment flakes or platelets exhibit a poor color shift and low color purity. Maximum color shift and color purity requires the optically variable pigment flakes or platelets in the ink or coating to adopt a same particular orientation, e.g. coplanar to the surface of the substrate.
These optical effects are even more enhanced if the surface of the substrate has been previously smoothened through the application of a base coating. The optically variable pigment flakes can in this case arrange more readily in a flat plane, thus increasing the coverage on the substrate, the color purity and the color shift.
To obtain coatings having the optically variable pigment flakes arranged in a same flat plane on a substrate, an ink or coating formulation which allows for a reduction of the wet film thickness during the drying process to less than 10 μm is normally used. The gradual reduction of the film thickness during the drying process forces the optically variable pigment flakes to arrange in a single plane parallel to the surface of the substrate, providing for a maximum coverage and color shift on the substrate.
Magnetic optically variable pigments have been disclosed in WO 02/073250; U.S. Pat. No. 4,838,648; EP-A-686675; WO 03/00801 and U.S. Pat. No. 6,838,166 as an improvement to the optically variable pigments for inks on security-, value-documents and banknotes; these documents are incorporated herein by reference.
Magnetic optically variable pigments in printing inks or coatings allow for the production of magnetically induced images, designs and/or patterns through the application of a corresponding magnetic field, causing a local orientation of the magnetic optically variable pigment in the coating, followed by drying/curing the latter. The result is a fixed magnetically induced image, design or pattern in an optically variable ink.
Materials and technology for the orientation of magnetic particles in coating compositions and printing processes have been disclosed in U.S. Pat. No. 2,418,479; U.S. Pat. No. 2,570,856; U.S. Pat. No. 3,791,864; DE 2006848-A; U.S. Pat. No. 3,676,273; U.S. Pat. No. 5,364,689; U.S. Pat. No. 6,103,361; US 2004/0051297; US 2004/0009309; EP-A-710508, WO 02/090002; WO 03/000801; WO 2005/002866, and US 2002/0160194; these documents are incorporated herein by reference.
U.S. Pat. No. 2,418,479 and U.S. Pat. No. 2,570,856 disclose a process and a coating composition for the magnetic orientation of metallic pigments in paint films. Said films have a high degree of orientation and a low degree of brightness, as well as unusual reflectance and translucency properties. The metallic pigments comprise flakes of ferromagnetic material, preferably nickel, in amounts ranging from 0.1% to 95% by weight of the film forming binder; and volatile organic components are present in the composition in amounts ranging from 50% to 70% of the total weight. The wet film is applied in a thickness of 25 mils (635 μm), and subject to a magnetic field for orienting the metallic flakes, maintaining the field until the film is dry. These documents are not concerned with OVI® and disclose merely paint compositions comprising magnetic metal flake pigments and effect coatings based thereupon. No formulation rules are given as to the relationship of flake size, flake concentration and coating thickness for obtaining the best optical effect.
U.S. Pat. No. 3,791,864 and DE 2006848-A refer to stove-enamel compositions, nitro-cellulose compositions, and two-component compositions comprising magnetic components (e.g. lamellar or rod shaped iron pigments), for the production of magnetically oriented coatings. The documents are about a method and a process for the magnetic orientation of pigments in two layer coatings; the formulation aspects of the involved coating composition are not, however, contemplated.
U.S. Pat. No. 3,676,273 discloses a magnetically oriented coating, comprising highly reflective nickel flakes dispersed in an acrylic binder. The amount of magnetic pigment ranges form 0.1% to 95% by weight of the film forming material. Specific formulation aspects are not addressed in this document.
U.S. Pat. No. 5,364,689 discloses a painted product comprising magnetic non-spherical particles in a paint medium, wherein the said magnetic particles are oriented to yield a pattern of a three-dimensional optical appearance. The magnetic particles comprise one or more of nickel, cobalt, iron, and their alloys. The particles have a thickness of 0.1 to 1.0 μm and a length of 10 to 100 μm. The paint medium is selected from alkyd, polyester, acrylic, polyurethane, and vinyl resins. The particles are present in amounts between 1 and 60 parts per 100 parts by weight of the paint medium. No particular rules concerning the formulation of the paint medium are given, however.
U.S. Pat. No. 6,103,361 refers to heat resistant coating compositions containing fluoropolymers, such as PFTE (poly-tetrafluoroethylene) and magnetizable flakes, which allow for the magnetically inducing of a pattern in the coating of a frying pan.
US 2004/0051297 and US 2004/0009309 refer to a method and an apparatus for orienting magnetic flakes during a painting or printing process. Particles of a magnetic optically variable pigment are dispersed in a liquid paint or ink medium. The typical flake is about 20 μm large and about one μm thick. The flakes typically include a magnetic metal layer such as a thin film of ferromagnetic metal or alloy, such as cobalt, nickel or PERMALLOY (typically 80% Ni, 20% Fe) and an optical interference structure, such as an absorber-dielectric-reflector Fabry-Perot type structure on both sides of the metal layer. US 2004/0051297 contains a remark as to the influence of film thickness and the type of organic carrier used on the magnetic orientability of the pigments. However, no further details concerning the best formulation of the coating composition for the application purpose are disclosed.
WO 02/090002 refers to methods for producing imaged coated articles by using magnetic optically variable pigment, as well as to coated articles. The pigment consists of reflective magnetic flakes (RMF) of the type described in WO 03/000801 “MultiLayered Magnetic Pigments and Foils” and has a magnetic core layer. No formulation rules are given, however, as to the coating composition which should be used.
WO 05/002866 refers to a method and means for producing a magnetically induced design in a coating containing magnetic particles. Said coating comprises preferably magnetic optically variable particles. The coating composition is preferably selected from the group of liquid inks, comprising screen-printing inks, gravure inks and flexographic inks. Liquid inks have low viscosity (in the range of 0.1 to 5 Pa*s at 20° C.) and allow for an easy orientation of the magnetic pigment. Drying/curing of the ink can be based on solvent or water evaporation, as well as on UV-crosslinking or on hybrid curing mechanisms, including evaporation of diluents, UV-curing and other reticulation reactions, such as oxypolymerization cross-linking reactions. None of the given ink formulas were optimized, however, with respect to the magnetic image/effect imprinted in the coating.
US 2002/0160194 refers to multi-layered magnetic pigments and foils. The disclosed pigment flakes can be interspersed within a binder medium to produce a colorant composition (ink) which can be applied to a wide variety of objects or papers. The binder medium contains a resin or mixtures of resins, and a solvent (organic solvent or water), and can be dried/cured by thermal processes such as thermal cross-linking, thermal setting, or thermal solvent evaporation, or by photochemical cross-linking.
The optically variable inks and coating compositions used in the art are aimed at exhibiting a bright color, a strong color shift and yielding a good substrate coverage using an as low as possible amount of optically variable pigment. A low pigment concentration is desirable to limit the raw material costs, as well as to obtain a good printability of the ink and durability of the print. These objectives are achieved by providing the printing inks with a relatively high amount of volatile components such as organic solvents, water or mixtures thereof, of the order of 50% or more, per weight of the composition, and a relatively low amount of non-volatile components, i.e. the binder medium and the OVP, of the order of 50% or less, per weight of the composition.
This particular formulation ensures a volume reduction of the coating layer during the drying process and a corresponding orientation of the OVP particles in the plane of the printed substrate. This is why most OVI formulations or coating formulations containing optical effect pigments are solvent or water based, with solid contents not exceeding 50%. The solid content represents the part of non-volatile compounds of a printed ink or coating layer after the drying/curing process.
In the case of magnetic optically variable pigments however, it has been found that this type of ink formulations, when used for the magnetic induction of images, patterns or designs in the printed ink layer, leads to poor visual effects.